As the recent semiconductor technique of the integrated circuit (IC) in quick developing, recent electronic products are trended to be thin and light and small for attracting people. In contrast to the recent electronic products, each of the past electronic products has a power supply with larger volume and cumbersome weight. The power supply is a type of linear power supply which has an isolation transformer, a heat dissipating sheet and a cooling fan with cumbersome weight. However, the switching power supply can overcome these problems and can achieve the advantages of high efficiency, light weight and small volume. Nevertheless, the switching power supply is more complex than the linear power supply, so stronger knowledge based on designs of analog electronic, magnetic member, logic and control is required to plan the design of the switching power supply.
In the switching type architecture of the AC/DC conversion, the power supply is usually needed to prevent Bulk line voltage from the occurrence of over voltage, for generally concentrating on safety. In particular to protect inner components of some special application, maximum duty is also limited in response to the detection of the Bulk line voltage. Further, with the development of technology, more and more functions are integrated in one IC. The requirement of the IC with low pins is larger and larger as well. In general technique, the functions of bulk line over voltage and the maximum duty limited by detecting of the Bulk line voltage are made in different design with different applied occasions. Generally, when the IC requires the functions of bulk line over voltage and the maximum duty limitation, two detection terminals are needed to set in the IC for protections of over voltage and maximum duty limitation. Hence, the pin quantity of the IC will be increased, and the occupied area of the IC will further increased.
Thus, a novel control circuit for controlling the power supply is provided to overcome above problem. The novel control circuit will detect two independently adjusted protection circuits via one pin of the IC for preventing the IC from the increment of the pin quantity, and for further reducing the occupied area of the IC.